At the present time, the majority of note-type personal computers, mobile telephones and the like are presently driven by a non-aqueous electrolyte secondary battery having a high voltage and a high energy density and showing an excellent self-discharge property. In this non-aqueous electrolyte secondary battery, carbon is frequently used as a material forming a negative electrode, and various organic solvents are used as an electrolyte for the purpose of reducing a risk when lithium is formed on a surface of the electrode and making a driving power high. Also, an alkali metal (particularly lithium metal or lithium alloy) or the like is used as a material for a negative electrode in a non-aqueous electrolyte secondary battery for a camera, so that an aprotic organic solvent, usually an ester based organic solvent or the like is used as an electrolyte.
These non-aqueous electrolyte secondary batteries are high in the performances, but have the following problems for the safety. Firstly, when the alkali metal (particularly lithium metal or lithium alloy) is used as the material for the negative electrode in the non-aqueous electrolyte secondary battery, since the alkali metal is very active for water, if the sealing of the battery is incomplete and water penetrates thereinto, there is a problem that a risk of generating hydrogen or firing is made high by reacting the material for the negative electrode with water. Also, since the lithium metal is low in the melting point (about 170° C.), there is a problem that if a large current violently flows in the short-circuiting or the like, the battery abnormally generates heat to cause an extremely risky state of fusing the battery or the like. Further, there is a problem that the electrolyte based on the organic solvent is vaporized and decomposed accompanied with the heat generation of the battery to generate a gas or the explosion and ignition of the battery are caused by the generated gas.
On the other hand, there are known a non-aqueous electrolyte secondary batteries in which a specified phosphazene derivative is added to the electrolyte to largely reduce the risk such as the ignition of the electrolyte or the like (see JP-A-6-13108 and JP-A-2002-83628). In these batteries, a self-extinguishing property or a flame retardance is given to the non-aqueous electrolyte by a nitrogen gas or a gas of phosphoric acid ester derived from the phosphazene derivative, whereby the risk of the ignition or the like is reduced. Also, phosphorus constituting the phosphazene derivative has an action of suppressing the chain decomposition of a high molecular weight material constituting the battery, so that the risk of the ignition or the like is effectively reduced.
However, cyclic phosphazene derivatives disclosed in JP-A-6-13108 and JP-A-2002-83628 are poor in the ability dissolving a support salt, so that when a greater amount of the cyclic phosphazene derivative is added to the electrolyte and a lithium salt is used as a support salt, an electric conductivity of lithium ion in the electrolyte is lowered to lower the electric conductivity and hence the quick discharge characteristic and quick charge characteristic of the battery are poor. Recently, a quick start (quick discharge) characteristic or an energy recovery (quick charge) characteristic in the braking is demanded in actively examined non-aqueous electrolyte secondary batteries for electric cars, so that the battery having the cyclic phosphazene derivative added to the electrolyte has a problem as a battery for the electric car. Also, such a tendency becomes conspicuous at a temperature lower than room temperature, so that there is a problem in the quick discharge and quick charge characteristics under a lower temperature environment.
On the other hand, chain phosphazene derivatives disclosed in JP-A-6-13108 and JP-A-2002-83628 are sufficient in the ability dissolving the support salt, but are somewhat higher in the viscosity as compared with the cyclic phosphazene derivative, so that when such a chain phosphazene derivative is added to the electrolyte, there is a tendency of lowering the electric conductivity of the battery. The lowering of the electric conductivity results in the lowering of the above quick discharge and quick charge characteristics, so that the battery having the chain phosphazene derivative added to the electrolyte has a problem in the quick discharge and quick charge characteristics.